Many modern electronic systems incorporate various electronic components. For example, personal computers may include electronic components in the form of dynamic random access memory (DRAM) units. Each of the electronic components may include an integrated circuit fabricated on a substrate.
The techniques used in fabricating the integrated circuit on a substrate and packaging it for use with the electronic system are many. As a part of one approach, photolithography may be used to place geometric patterns that define devices onto the substrate surface. The photolithography process may utilize a projection-type aligner.
In a projection type aligner, known as a stepper, an illuminating light is irradiated onto a reticle or the original of a mask on which the patterns have been formed for the formation of a given circuit. The projected image of the pattern is transferred by exposure to a photoresist-coated layer on the substrate. Most of the process may be automated. For example, after a reticle case and a wafer carrier are mounted, the stepper may automatically perform the following functions according to the data and sequence stored in a data file: reticle transport to a reticle stage, wafer transport to the wafer stage, reticle pattern focusing on a wafer surface, reticle and wafer alignment, step and repeat of wafer stage, exposure, wafer transport, and the transport of the next wafer to the wafer stage.
In more recent times, as the pattern dimensions of integrated circuits have increasingly become smaller with higher densities, increased demands have been placed on the optical conditions of the stepper such as the focus for the transfer. A small particle or contaminant on a substrate-supporting chuck for holding the substrate during the photolithography exposure process may cause a number of problems, e.g., inexact focusing that may cause the integrated circuit to fail. Because a particle may cling to the substrate-supporting chucks between the chuck and a substrate being exposed on a stepper, any defect caused by the particle may continue until the next cleaning of the chucks. The delay in cleaning may cause as much as a day's worth of material to be bad. On the other hand, if the routine maintenance of cleaning the chucks is performed unnecessarily, time and resources may be needlessly applied.
Cleaning the chucks frequently involves physically cleaning the chucks with isopropyl alcohol. Even with this cleaning process, it is not guaranteed that all the particles will be removed, and in fact, on occasion, a particle may be added by the operator cleaning the chucks.
One approach to addressing the particle-on-the-substrate problem has been to form pin chucks. Pin chucks have a plurality of pins used to support a substrate. If a particle finds its way to the chucks, it will ideally be located between the pins such that the particle will not cause the substrate to be unlevel with respect to a desired exposure plane. The contamination event is, however, not detected with a pin chuck. Furthermore, as dimensions continue to decrease on integrated circuits while wafer sizes continue to increase, this approach may become more limited.
Another approach in trying to correct for particle contamination of substrate-supporting chucks is to provide for focus compensation. Using this approach, compensation is based on information gathered during the tools processing of the wafer. The approach is limited to the size of the area exposed at a given time and is based on the average value of the focus.